1. Field of the Invention
The present invention relates to a carbon fiber, and, more particularly, to a high elongation, high strength pitch-type carbon fiber which can be easily handled and thereby easily knitted and woven and a manufacturing method therefor. The high elongation, high strength pitch-type carbon fiber according to the present invention can be widely used as a reinforcing fiber for light-weight structural material employed in the space, automobile and architecture industries.
2. Related Art Statement
Hitherto, although PAN-type carbon fibers and rayon-type carbon fibers have been widely manufactured and used, both the PAN-type carbon fiber and the rayon-type carbon fiber have a problem in terms of the cost thereof because they consist of materials which are too expensive and have poor carbonization yield. Accordingly, pitch-type carbon fibers have attracted special interest because they are made of pitch which is inexpensive, and they exhibit excellent tensile strength and tensile elastic modulus.
At present, the pitch-type carbon fiber has been manufactured as follows:
(1) Carbonaceous pitch suitably used to manufacture the carbon fiber is prepared from petroleum pitch or coal pitch so as to be heated and melted before it is spun by a spinning machine so that a pitch fiber bundle is manufactured by collecting and doubling the fibers; PA0 (2) The pitch fiber bundle thus manufactured is heated up to 200.degree. to 350.degree. C. in an atmosphere of an oxidizing gas in an infusible furnace so as to be infusibilized; and PA0 (3) Then, the fiber bundle thus infusibilized is heated up to 500.degree. to 2000.degree. C. in an atmosphere of an inert gas so as to carbonize it before it is further heated up to 3000.degree. C. so as to graphitize it.
The pitch-type carbon fiber thus manufactured exhibits an excellent tensile strength of 2.0 GPa (200 kg/mm.sup.2) or more and tensile elastic modulus of 600 GPa (60 ton/mm.sup.2) or more. However, it has been suffered from unsatisfactory elongation of 0.5% or less in usual, the same being about 1% at the most.
As described above, the elongation of the conventional pitch-type carbon fiber is insufficient to be easily handled. As a result, it cannot be easily knitted and woven, causing a critical problem to be arisen in that an excellent composite material cannot be easily manufactured.
From a study for manufacturing a high elongation pitch-type carbon fiber, the inventors of the invention have found a fact that a pitch-type carbon fiber exhibiting a satisfactory tensile strength and a tensile elastic modulus and as well exhibiting an elongation of 1.0% or more, which enables an excellent knitting and weaving facility to be obtained, can be manufactured from the pitch with maintaining the satisfactory tensile strength and the tensile elastic modulus The above-described pitch-type carbon fiber can be realized by arranging the crystalline structure to be a specific form. That is, in the specific crystalline structure of the present fiber the orientation angle (.phi.), stack height (Lc) and interlayer spacing (d.sub.002) of the X-ray structural parameter are 25.degree. to 38.degree., 19 to 35 .ANG. and 3.45 to 3.50 .ANG., respectively.
Further, the inventors have found a fact that the adhesive property between the fiber and the matrix resin, which is the most critical factor when a composite material is manufactured from a carbon fiber, considerably depended upon the surface oxygen content of the carbon fiber and the total oxygen content in the whole of the carbon fiber. That is, the adhesive property between the fiber and the matrix resin becomes satisfactory in the case where the atomic ratio (O/C) of oxygen to carbon on the surface of the fiber measured by a X-ray photoelectron spectrometry is 0.1 to 0.35 and the total oxygen content in the whole carbon fiber is 0.01 to 0.2 wt. %. It was found that if the atomic ratio (O/C) of oxygen to carbon on the surface of the fiber is less than 0.1 and the total oxygen content in the carbon fiber is less than 0.01 wt. %, the adhesive property might be excessively deteriorated. Furthermore, it was found that if the atomic ratio (O/C) of oxygen to carbon on the surface of the fiber exceeds 0.35 and the total oxygen content in the carbon fiber exceeds 0.2 wt. %, the tensile strength and the tensile elastic modulus of the carbon fiber deteriorate excessively.
Furthermore, the inventors of the invention found a fact that the above-described novel high elongation and high strength pitch-type carbon fiber can be manufacture by applying a predetermined tention at the time of the carbonization process subjected to the infusibilized fiber and quickly carbonizing the fiber within a range in which the fibers can not be melted and adhered to each other. Furthermore, the adhesive property with the matrix can be improved and the physical property of the fiber can also be improved when the fiber is subjected to oxidation after carbonization.
Thus, the above-described newly findings cause the present invention to be established.